In a normal setup a Radio Access Network node, such as a base station controller BSC/radio network controller RNC node, is typically connected to a Core Network node, such as a mobile switching center MSC, in a one-to-one relationship. Within a MSC in pool setup, sometimes abbreviated MiP, there is no need for such a one-to-one relationship but a Radio Access Network node may be connected to a plurality of Core Network nodes. Such a configuration allows for load balancing and provides improved reliability as a node failure of a Core Network node may be remedied by a fallback Core Network Node.
An exemplary configuration of a MSC in pool setup is shown in FIG. 1. In this MiP example, a plurality of radio access nodes, such as BSC 1, BSC 2 and/or BSC 3, may be connected to a plurality of core network nodes, such as MSC 1, MSC 2 and/or MSC 3 in any kind of relationship. I.e. in the shown scenario each MSC may communicate with any BSC, thereby allowing for performing call handling.
If no failure of a core network node is experienced, a typical Mobile Terminating call scenario in MiP: as shown in FIG. 2 with respect to a mobile terminating call handling in the GSM MiP network is happening. It is noted, that the handling in WCDMA as well as other radio networks is quite similar and therefore not shown.
In FIG. 2 it is assumed that a radio access network node BSC is connected to a plurality of core network nodes MSC 1, MSC 2, MSC 3. A Mobile Station MS of a subscriber is registered in MSC 1. Now if a call from an originating node is to be established towards said Mobile Station MS, at a certain point in time within the call processing a home location register HLR storing information pertaining to said subscriber is contacted, e.g. by a mobile application part MAP message such as Send Routing Information, sometimes abbreviated SRI, see step 1. The home location register HLR dispatches a message requesting a roaming number towards the MSC in which the Mobile Station is registered, i.e. MSC 1, e.g. a MAP message Provide Roaming Number, sometimes abbreviated PRN, see step 2.
Having received said PRN message, MSC 1 allocates a Mobile Station Roaming Number MSRN for the concerned subscriber and returns the PRN message to the home location register HLR, see step 3. In turn, the home location register HLR, returns a SRI message towards the originating core network node comprising the allocated Mobile Station Roaming Number MSRN, see step 4.
In response, the originating core network node issues an Initial Address Message, sometimes abbreviated IAM, towards the MSC 1, see step 5. The MSC 1 pages the concerned Mobile Station MS by sending a paging request, e.g. BSSMAP message Paging Request comprising a Temporary Mobile Subscriber Identity TMSI of the concerned Mobile Station MS, towards a respective Radio Access Network node BSC, see step 6.
The Radio Access Network node BSC sends a Paging message comprising said TMSI towards the Mobile Station MS, see step 7. The Mobile Station MS, in turn, derives the Intra-Domain NAS Node Selector parameter, sometimes abbreviated IDNNS, from its TMSI, see step 8. In response, the Mobile Station MS returns a Paging Response comprising IDNNS parameter and an encapsulated DTAP message Paging Response, see step 9.
The Radio Access Network node BSC selects a Core Network node, i.e. MSC 1, based on the received IDNNS parameter, see step 10 and sends a BSSMAP Complete Layer 3 message, which encapsulates DTAP message Paging Response, see step 11. The Core Network Node MSC 1, in turn, continues the terminating call setup, see step 12.
However, if an MSC within a MSC Pool is failing, the mobile terminating traffic towards the subscriber will be unsuccessful unless the Mobile Station MS is used for attempting a mobile originating call and gets registered in another MSC of said MSC Pool or in case of a periodic Location Update.
Recently, ideas have been proposed allowing to detect such a failure within a first attempt of a mobile terminating call and to trigger corrective actions. However, these solutions on the benefit that future mobile terminating calls may be successful always lead to a situation where the first mobile terminated call will fail.
A respective idea is shown in some details in FIG. 3. In FIG. 3 it is assumed that a radio access network node BSC is connected to a plurality of core network nodes MSC 1, MSC 2, MSC 3. A Mobile Station MS of a subscriber is registered in MSC 1. MSC 2 is configured as a fallback MSC for MSC 1. Hence, MSC 1 is providing subscriber data such as International Mobile Subscriber Identity, sometimes abbreviated IMSI, and Location Area Identifier towards its fallback MSC, i.e. MSC 2, see step 1. The fallback MSC is the MSC that shall handle the Mobile Terminating MT traffic for the failed MSC within the MiP. At a certain point in time indicated by the cross MSC 1 is failing.
Now if a call from an originating node is to be established towards said Mobile Station MS, at a certain point in time within the call processing a home location register HLR storing information pertaining to said subscriber is contacted, e.g. a MAP message such as Send Routing Information, sometimes abbreviated SRI, see step 2. The home location register HLR dispatches a message requesting a roaming number towards the MSC in which the Mobile Station is registered, i.e. MSC 1, e.g. a MAP message Provide Roaming Number, sometimes abbreviated PRN, see step 3. Since MSC 1 is failed, the message is rerouted towards the fallback MSC, i.e. MSC 2.
By checking the received IMSI as provided within the PRN message, MSC 2 determines that an MSC for which MSC 2 is configured as a fallback MSC is failed. Consequently, MSC 2 returns a PRN message with no roaming number available. Hence, the first mobile terminated call fails, see step 4.
In order to allow for a call processing for a next mobile terminated call, MSC 2 pages the concerned Mobile Station MS by sending BSSMAP message Paging Request with IMSI and the stored LAI towards the Radio Access Network BSC, see step 5a. 
For the current call, the HLR responds to the service request with a negative result thereby the first mobile terminated call after failure will fail, see step 5b. 
The BSC sends a Paging Request message with IMSI towards the Mobile Station MS, see step 6. After receiving paging request, the Mobile Station MS derives the IDNNS parameter from its IMSI and returns a Radio Resource Control RRC paging response message, see step 7.
The BSC, in turn, will forward this paging response message to MSC 2 as the originator of BSSMAP Paging Request with IMSI, see step 8. On reception of the success paging response, MSC 2 triggers a TMSI re-allocation procedure using the Non Broadcast Location Area Identifier which is unique for each MSC within the pool, sometimes abbreviated NB-LAI, and a TMSI comprising a Null-NRI, see step 9.
After receiving the TMSI re-allocation response from said Mobile Station MS, MSC releases the resource on the radio interface, see step 10. The Mobile Station MS performs a Location Update towards the network, see step 11. Based on some decision criteria, such as a capacity factor in MiP, the BSC forwards this Location Area request to MSC 3, see step 12.
Now, the Mobile Station MS is registered in MSC 3 and gets respective subscriber information from HLR in step 13.
Now if another call from an originating node is to be established towards said Mobile Station MS, at a certain point in time within the call processing a home location register HLR storing information pertaining to said subscriber is contacted, e.g. a MAP message such as Send Routing Information, sometimes abbreviated SRI, see step 14. The home location register HLR dispatches a message requesting a roaming number towards the MSC in which the Mobile Station is now registered, i.e. MSC 3, e.g. a MAP message Provide Roaming Number, sometimes abbreviated PRN, see step 15.
Since now the MSC 3 is not failed, this further mobile terminated call will be successful. This situation is not only experienced in GSM based networks, but as well in UMTS and LTE networks.
I.e. in CSFB (Circuit Switched Fallback) scenario within LTE, if a MSC in the pool fails, the respective MSC will/cannot send indication to Mobility Management Entity MME. As a consequence, the parameter VLR reliable in the core network node Mobility Management Entity, sometimes abbreviated MME, will be kept true, as from MME's point of view the SGs association of MME and MSCs towards the failed MSC is still kept until a combined Tracking Area Update Request (TAU) from the User Equipment (UE) is received. Upon the next TAU, MME will select another available MSC in the pool. The first MT CSFB call will always fail before the subscriber makes mobile originating attempt and gets registered in a new MSC Pool member or a periodic TAU.
A like situation is also experienced with respect to other services such as SMS service towards a Mobile Station being handled by a failing core network node within a pool of core network nodes.